1. Field of the Invention
The present invention relates to methods for agglomerating recycled fines of silicon/silicon carbide resulting from the cutting of silicon ingots. More particularly, the present invention relates to silicon/silicon carbide agglomerations for use in metallurgical processes.
2. Description of Related Art
Silicon wafers used in the production of photovoltaic solar cells and/or microelectronics are prepared from silicon ingots. When the silicon wafers are cut there is a major loss of silicon during the cutting process. As technology for cutting thinner wafers develops, there is more waste material produced. Reciprocating wire saws contact the silicon ingot being sliced while a liquid slurry containing abrasive grains, such as silicon carbide, is supplied to a contact area between the silicon ingot and the wire saw. Used slurries from the wire saws are liquid suspensions of abrasive particles-silicon carbide or diamond grit-and abrasion products of the sawing operation, i.e., steel residue from the saw wires and silicon kerf from the silicon ingot. The liquid phase of the slurry is an organic or aqueous medium fulfilling the dual function of keeping the abrasive material in suspension and cooling the cutting zone between the wire and the silicon ingot, usually polyethylene glycol.
The abrasive particles of the slurry are rubbed by the wire saw against the ingot surface as the ingot is sliced causing silicon particles from the ingot to be removed resulting in loss of silicon. In addition, the silicon carbide abrasive grains break down and the wire wears adding increased particles in the slurry. The removed silicon resulting from the sliced ingot is known as the “saw kerf.” As the concentration of silicon and silicon carbide (SiSiC) fines in the slurry increases, the efficiency of the slicing operation decreases. At some point, the slurry becomes loaded with SiSiC fines and is rendered ineffective, or “exhausted.” If any of the exhausted slurry is to be used further it needs to be recycled or reprocessed. Namely, the silicon carbide, silicon and iron (from the wire) fines need to be removed from the “good” silicon carbide and the polyethylene glycol (PEG). The removed fines are the starting material for the processes of this invention. Historically these fines have not been utilized.
Specifically, although wire saw technology has improved, each pass of the wire through the silicon ingot results in the loss of an amount of silicon equivalent to about a 250 micron thick slice of the ingot or less. As technology enables thinner and thinner wafers to be sliced from the ingot, more and more passes of the wire through the ingot occur, resulting in more and more loss of silicon.
The exhausted slurry may be treated by various technologies to separate the good silicon carbide and polyethylene glycol from silicon carbide fines, the wire fines and the silicon kerf. As noted, the exhausted slurry contains good silicon carbide, fine silicon carbide, silicon kerf, iron fines and PEG. The remaining solid particle fraction of fines cannot be reused for wiresawing. There have been efforts to recover the silicon fines for further use; however, this further recovery processing often proves too costly and the exhausted slurry waste product is then sent to a land fill. Of course, this method of disposal is unfavorable from an environmental point of view, as well as, the costs associated therewith. In addition to the environmental and economic concerns associated with the exhausted slurry, the loss of potentially useful silicon/silicon carbide material should also be considered.
SiSiC material is useful in steel making as it acts as a fuel providing energy to the steel making process and serves as an effective slag deoxidizer. However, the SiSiC found in the abrasion slurries cannot be used in that form and must be processed to place the SiSiC in a form that can be used to economically make steel. Therefore, there is a need for a method to utilize the SiSiC created by the cutting of silicon ingots into a form so that the recovered SiSiC can be used in various applications, such as iron and steel melting, and other metallurgical processes using SiSiC addition. Thus, the possibility of utilizing recovered SiSiC material produced during silicon wafer production versus only utilizing silicon via costly and difficult methods is a major advantage of the present invention.